I. Field of the Invention
The invention applies to video signal interface apparatus, and more particularly to apparatus which provides a means of driving the column of an LCD active matrix with a variable voltage to produce grey scale but with only drivers which operate in a saturated mode.
II. Discussion of the Prior Art
State-of-the-art analog column drivers used in active matrix LCD grey scale applications dissipate power at a high rate. This high power dissipation limits the temperature operating range of the active matrix, reduces reliability and limits maximum panel size and pixel density.
FIG. 1 shows a conventional grey scale liquid crystal display (LCD) column drive circuit in block diagram form. In such conventional circuits, a logical "1" is entered into a shift register 10 and propagated therein to produce one single-column address at a time. This is done sequentially until all columns have been addressed. Each time a column is addressed, the appropriate switch S is activated and a sample-and-hold capacitor C1A, for example, is selected to store the video voltage. During this setup, capacitor C1B which was accessed one line earlier by input switch B1 is providing the video voltage for that column for the particular row currently being output through output switch A2 and an analog line driver 20 to the display. There are several variations on this theme, such as using a digital-to-analog converter (DAC) to store voltages instead of a capacitor in a keyed sample-and-hold circuit as described above. Such circuits require high power consumption and are very complex in comparison to the invention. Such complexity and high power consumption might be warranted if a larger number of grey levels were available. However, a typical LCD display is limited by construction and viewing angle variation to a small number of grey shades. For the example shown, 16 grey shades are used. In such cases, the extra power and increased complexity of more capable column drivers is not warranted.